ZTEST function
Returns the onetailed probabilityvalue of a ztest. For a given hypothesized population mean, μ0, ZTEST returns the probability that the sample mean would be greater than the average of observations in the data set (array) — that is, the observed sample mean.
To see how ZTEST can be used in a formula to compute a twotailed probability value, see "Remarks" below.
Important This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. This function is still available for compatibility with earlier versions of Excel. However, if backward compatibility is not required, you should consider using the new functions from now on, because they more accurately describe their functionality.
For more information about the new function, see Z.TEST function.
Syntax
ZTEST(array,x,[sigma])
The ZTEST function syntax has the following arguments:

Array Required. The array or range of data against which to test x.

X Required. The value to test.

Sigma Optional. The population (known) standard deviation. If omitted, the sample standard deviation is used.
Remarks

If array is empty, ZTEST returns the #N/A error value.

ZTEST is calculated as follows when sigma is not omitted:
or when sigma is omitted:
where x is the sample mean AVERAGE(array); s is the sample standard deviation STDEV(array); and n is the number of observations in the sample COUNT(array).

ZTEST represents the probability that the sample mean would be greater than the observed value AVERAGE(array), when the underlying population mean is μ0. From the symmetry of the Normal distribution, if AVERAGE(array) < μ0, ZTEST will return a value greater than 0.5.

The following Excel formula can be used to calculate the twotailed probability that the sample mean would be further from μ0 (in either direction) than AVERAGE(array), when the underlying population mean is μ0:
=2 * MIN(ZTEST(array,μ0,sigma), 1  ZTEST(array,μ0,sigma)).
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article.
Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In Excel, create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V.
Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

